Exploring decomposition mechanism of Hydroxylamine on Cu(111) and Iridium(111) surface using Pynta – Workflow code for heterogeneous catalysis reaction

Shinae Kim¹, Matthew S. Johnson¹, Trevor Price², Judit Zádor¹

¹Combustion Research Facility, Sandia National Laboratories, Livermore, CA, USA

²Department of Chemical engineering, University of California - Davis, Davis, CA, USA

Hydroxylammonium nitrate (HAN) is a promising monopropellant that can replace toxic and highly explosive hydrazine-based monopropellants. Previously, mass spectroscopy results showed that HAN decomposes to hydroxylamine (HA)¹; the catalytic decomposition reaction of HA creates critical ignition conditions in HAN-based propellants. While copper is widely used for propellant catalysts, the mechanisms for the catalytic decomposition reaction of HA on copper surfaces have not yet been examined. In addition to copper, current efforts to use iridium-based catalysts motivated us to explore HA decomposition reactions on the iridium (111) surface.

Heterogeneous catalytic reactions of HA on copper and iridium surfaces are examined using Pynta² (https://github.com/zadorlab/pynta), a recently developed automated workflow code for calculating thermochemical and kinetic parameters for surface and gas surface reactions. Pynta can automatically set up and process a large number of quantum chemistry calculations to study the reactions of adsorbates and gas-phase species on metal facets. Using the harmonically forced saddle point searching method, Pynta can generate chemically sensible and comprehensive initial guesses for saddle point geometries by defining harmonic potentials based on the optimized adsorbate geometries and bonds that are forming and breaking during the reaction.

We demonstrate that Pynta is able to identify transition states and generate rate coefficients for possible HA decomposition pathways. Along with kinetic and thermodynamic parameters, the newly implemented bonding analysis module using quasi-atomic orbitals (QUAO) in Pynta can identify changes between the metal surfaces and adsorbates, as well as the relative strengths of the metal-hydrogen bonds forming between HA and the metal surfaces.

[1] Chambreau, S.D.; Popolan-Vaida, D.M.; Vaghjiani, G.L.; Leone, S.R., J. Phys.Chem. Lett. 2017 8(10), 2126-2130

[2] Johnson, M.S.; Girerada, M.; Hermes E.D.; Bross D.H.; Sargsyan, K.; Najm, H.N.; Zador J., J. Chem. Inf. Model. 2023, 63(16), 5153-5168